Young men who have sex with men (YMSM) in the U.S. have a disproportionally higher incidence of Human Immunodeficiency Virus (HIV) and other sexually transmitted infections (STIs) compared with other MSM. These men also have low uptake of HIV/STI testing and pre-exposure prophylaxis (PrEP), a highly efficacious HIV prevention strategy when taken daily. Known barriers include low perceived HIV risk, difficulty accessing providers, lack of engagement in regular healthcare, stigma, and concerns about confidentiality. The high uptake of mobile devices among youth provides an opportunity to leverage mobile health applications on hand- held devices (mHealth) to increase HIV/STI testing and PrEP uptake among YMSM. The major goals of this pilot project are to refine and pilot test a combination mobile app based intervention designed to overcome barriers to HIV/STI testing and PrEP uptake among YMSM. Using qualitative methods and informed by the Information, Motivation, and Behavioral Skills theoretical model, we will build upon an existing mobile app platform to develop and tailor the mobile app, LYNX, to increase both HIV/STI testing and PrEP uptake among YMSM through an iterative development design method (aim 1). We will then pilot test this mobile app for usability in a 2-month open technical pilot (aim 2). After revising this app, we will subsequently conduct a 6-month pilot randomized controlled trial to evaluate the acceptability and feasibility of LYNX among a diverse sample of YMSM in Chicago, IL and Tampa, FL. Upon completion of this 2-year project, we will have optimized the LYNX mobile app for increasing HIV/STI testing and PrEP uptake among YMSM, which will be poised for further evaluation in a full-scale efficacy trial in years 3-5 of the iTech.